re:
Every page type also has a database table corresponding to it. Every time we modify the database, we need to rebuild it. We can do this by going to http://localhost/dev/build?flush=1. It may take a moment, so be patient. This add tables and fields needed by your site, and modifies any structures that have changed. It does this non-destructively - it will never delete your data.

As we have just created a new page type, SilverStripe will add this to the list of page types in the database.

in particular: http://localhost/dev/build?flush=1

I am trying to achieve this on a online site. but it seems to come up with a 'page not found'

using path: http:/my_site.com/sapphire/dev/build?flush=1

I can;t seem to get it to work for some reason.